The present invention relates to acoustic wave devices and more particularly to acoustic wave storage devices for convolutely combining two high frequency signals.
Devices for performing the convolution or correlation operation on a pair of signals f.sub.1 and f.sub.2 are well known in the art. Such devices, typically referred to as convolvers or correlators, find application in various disciplines particularly in the communications field. For example, it is known that convolvers are the essential circuit elements in systems designed for removing the distortion produced by multipath transmission in communication channels. In television reception, multipath transmission effects are observably manifested by the production of replica images, i.e., "ghosts," displaced on the viewing screen of a television receiver from the main image. Apparatus contained within the television receiver for reducing the effect of such ghosts are known as ghost cancellation systems and normally include a convolver as the essential part thereof. In this regard, ghost cancellation is achieved by convolutely processing a reference signal transmitted during, for example, the vertical interval of the television signal, with the transmitted video signal to obtain a correction or synthetic ghost signal. The synthetic ghost signal is then suitably combined with the video signal to cancel the effect of ghosts therefrom.
It is well known in the prior art to employ various devices for the processing of two signals through the operation of convolution. U.S. Pat. No. 3,935,439 to Buss et al. discloses the use of charge transfer technology for this purpose.
Acoustic surface wave technology is another field in which the prior art workers have made extensive efforts to realize convolution arrangements. Such arrangements generally embody devices based on the non-linear parametric interaction of acoustic surface waves, see U.S. Pat. No. 3,794,939 to Waldner, or, alternatively, devices using semiconductor storage elements to facilitate the performance of the convolution operation on a pair of high frequency input signals previously transformed into surface waves. U.S. Pat. No. 3,975,696 to Kantorowicz typifies the prior art semiconductor storage type acoustic wave convolver.
Prior art acoustic wave storage convolvers are known in which a silicon substrate having a plurality of electrodes is brought into close proximity, for instance about 2,000 angstroms, to one surface of a surface wave propagating medium for providing a capacitive effect whereby charge is transferable to a large number of diodes fabricated on the silicon substrate. Charge is transferred to the diodes in response to the application of a read-in pulse applied across the air gap, this charge being modulated by propagating one of the signals to be convolved along the medium thereby storing this signal as changes in the capacitances of the storage diodes. The second signal to be convolved is subsequently propagated along the medium in a direction opposite of the first signal and interacts with the storage diodes to produce an output representing the convolution of the first and second signals.
Some of the problems associated with the fabrication and maintenance of the aforementioned uniform narrow air-gap, which makes possible uniform interaction between the silicon substrate and the surface wave propagating medium, are avoided by forming electrodes directly on the surface of the medium as taught in the previously mentioned Kantorowicz patent. However, other problems indigenous to known surface wave storage convolvers, e.g., the diode density per acoustic wave length and distortion producing coherent reflections from sampling elements, represent design constraints still imposing various undesirable restrictions on the use of such devices.
Conventionally, it has been considered necessary to use several diodes, usually at least three, per cycle or acoustic wavelength of the propagating signal in order to store a faithful representation of the signal. Due to the packaging density normally required to accommodate even three diodes per acoustic wavelength, the prior art has generally been limited to the use of integrated diode structures such as those fabricated on a semiconductor substrate as previously described. It would be desirable to improve on the prior art surface wave storage convolver in this respect by reducing to a minimum the number of diodes used to store a signal propagated along the surface of such a device. Among other factors, including reasons of economy, a reduction in diode density sufficient to enable the use of discrete diodes which could be selected for use in accordance with some predetermined criteria would significantly increase the flexibility of surface wave storage convolvers. In addition to the preselection facility, the discrete diodes could be individually matched and also replaced and/or repaired as necessary.